Field of the Invention
The present invention relates to circuits used with oxygen-ion concentration proportional sensors located in the exhaust system of an internal combustion engine and coupled to an air-fuel ratio control system for the internal combustion engine.
Description of the Prior Art
An oxygen-ion concentration proportional sensor that is well-known in the prior art. The sensor includes first and second elements that converge at a common wall to form a body of the sensor. The body defines therein a reference gas chamber that is open to a source of ambient air and an exhaust gas chamber that is coupled through a gas flow slit to the exhaust system of an internal combustion engine.
The body and wall elements are formed from a solid electrolytic material having oxygen-ion conductivity that may be formed from zirconium dioxide for example. Electrodes are formed on opposing sides of a common wall to constitute a pumping cell and on opposing sides of the body element to form a sensing cell. An electrical heater element is coupled to a surface of the body element for heating the entire structure to approximately 800.degree. Celsius in order to activate the entire oxygen sensor.
Under these conditions if a constant voltage is maintained across the electrodes of the sensing cell, then the pumping current flowing through the cells will be proportional to the oxygen content of the adjacent gases. Under operating conditions, the oxygen content of the adjacent gasses will vary with the changes in the oxygen content of the exhaust gasses produced by the internal combustion engine, and the pumping cell current is adjusted by the control electronics to maintain a constant sensing cell voltage. In this manner the resulting pumping cell current is a direct indication of the oxygen content in the exhaust gasses being monitored.
FIG. 1 illustrates a circuit for being used with the prior art EGO sensor described above. A high gain differential amplifier 10 includes a negative input 12, a positive input 14 and an output 16. The negative input 12 is connected to terminal c of the sensing cell 100 in the Exhaust Gas Oxygen sensor (EGO sensor) 100. The positive input 14 of the high gain differential amplifier 10 is coupled to a calibration voltage VCAL plus a bias voltage of 0.45 volts, while the output 16 is coupled through an electrical conductor to an input a of the pumping cell 101 of the EGO sensor 100.
A series precision 10 ohm resister 20 has one end coupled to terminal b of the pumping cell 101 and the other end coupled to terminal d of the sensing cell 112 in the EGO sensor 100. The two inputs 32 and 34 of a differential amplifier 30 having a gain of 50 are coupled across the resistor 20, and an output 36 of the differential amplifier 30 represents the output voltage "EGO out" representative of the oxygen content sensed by the EGO sensor. This output voltage is then coupled through an analog to digital converter 40 and into the electronic engine control system 42 of the vehicle.
The output 16 from the high gain differential amplifier 10, which is coupled through conductor 18 back to terminal a of the pumping cell 101 of the EGO sensor 100, closes an electrical feedback loop from the sensing cell 112 to the pumping cell 101. This feedback loop maintains the sensing cell voltage, which is equal to 0.45 volts in this example, by adjusting the pumping current (IP) from the output 16 of the high gain differential amplifier 10 to provide the required oxygen diffusion into the sensing cell 112. In normal operation the high gain differential amplifier 16 forces whatever pumping current is required through the conductor 18 in order to maintain the same voltage at the two inputs 12 and 14 of the high gain differential amplifier 10, which in the case of the preferred embodiment is equal to VCAL +0.45 volts.
For example, if the oxygen content in the exhaust gases were to be too high, which corresponds to the internal combustion engine running too lean, then the voltage from the sensing cell 112 would decrease at the input 12 to the differential amplifier 10. Since the second input 14 to the differential amplifier 10 is held constant, the decreasing voltage level at the first input 12 would result in the voltage at the output 16 going higher, thereby increasing the pumping current in conductor 18 in the positive direction. This in turn would cause the pumping cell 101 to pump oxygen away from the sensing cell 112, which would then bring the voltage at output terminal c of the sensing cell 112 and at the input 12 of the differential amplifier 10 back down to VCAL plus 0.45 volts.
With continuing reference to the prior art embodiment illustrated in FIG. 1, the pumping current IP exits the pumping cell 101 at terminal b and flows through the precision 10 ohm series resistor 20 which terminates at terminal d of the sensing cell 112. A differential amplifier 30 multiplies the voltage developed across resistor 20 by a linear factor of 50. The EGO output voltage at output 36 is then monitored by the input of the analog to digital converter 40 and the engine control system 42.
If the oxygen content in the engine exhaust gas is lower than the calibrated level, then more oxygen is pulled into the sensing cell 112 from the pumping cell 101 in order to maintain the 0.45 volt bias characteristic of the EGO sensor 100. If the oxygen in the exhaust gas is higher than the calibrated level, then the pumping cell 101 removes oxygen from the sensing cell 112 in order to maintain the 0.45 volt bias. The pumping rate, either in the positive or the negative direction, depends on the magnitude of the pumping current. Also, the direction of the oxygen pumping is controlled by the direction of the pumping current flowing through conduct 18.
In the embodiment illustrating the prior art as shown in FIG. 1, for an EGO output voltage that varies from 0 to 5 volts at the output 36 of the differential amplifier 30, and assuming that the calibration voltage at terminal d of the sensing cell 112 is equal to 2.5 volts, then for a pumping current equal to zero the EGO output from the output 36 of the differential amplifier 30 will be equal to 2.5 volts. This example assumes that resistor 20 is 10 ohm resistor and that the gain of the differential amplifier 30 is set to approximately 50.
In order for the circuit to operate under conditions corresponding to the lowest possible internal combustion engine emissions, it is necessary for the EGO sensor to operate with the output voltage from terminal b of pumping cell 101 (hereinafter called VP-) to be very close in potential to the input at terminal d of the sensing cell 112 (hereinafter VS-). In order to minimize the voltage drop between VP- and VS-, which under ideal circumstances should be zero, the resistance of resistor 20 should be reduced to a very low value. The prior art teaches that the maximum voltage across resistor 20 should be less than 50 millivolts (0.005 amps.times.10 ohms), but this requires that the resulting small signal voltage developed across resistor 20 be amplified by a factor of 50 in order to develop a sufficient full scale signal swing for optimum operation of the Analog to Digital Converter (ADC). Since the ADC can resolve voltages only as low as five to 10 millivolts, the signal at the output 36 of the differential amplifier 30 must be as large as possible in order for the system to detect small variations in the oxygen content of the exhaust gases. Amplification of the voltage developed across the sensing resistor 20 by a factor of 50 is prone to noise and errors, because any noise at the input terminals 32 and 34 of the differential amplifier 30, together with any offset voltages that may be present at the input terminals, will be amplified by a factor of 50 along with the signals. In a monolithic integrated circuit that integrates amplifiers 10 and 30, and the EGO heater control circuits, it would be impossible to completely isolate the noise generated by these circuits from the inputs 32 and 34 of the differential amplifier 30. The solution taught in the prior art is a compromise between how much noise and error can be tolerated by the differential amplifier 30, and how much error can be tolerated by allowing a voltage difference across the sensing resistor 20.
Another problem leading to performance degradation of the sensing system relates to that non-linear current and voltage characteristics (I-V curve) around the region where the pumping current is near zero. This region of operation is critical because it occurs when the engine is being controlled within the lowest emissions levels. In the voltage driven scheme for the pumping cell as used in the prior art, the output 16 of the amplifier 10 is a controlled voltage source. When the pumping cell goes through the non-linear region, a sudden change of voltage is required to change the pumping current just slightly. This requires the amplifier 10 to operate very quickly to cause the required change in voltage at its output 16. In reality, this change in output voltage takes a significant period of time due to finite slew rate of the internal amplifier nodes in the signal path being driven by the amplifier. During this delay, the pumping current is in error and the performance of the engine is degraded. If the pumping cell were driven by an amplifier with a current output and a replication of the pumping current could be generated in a circuit that is decoupled from the full loading effects of the pumping and sensing cells, then a more accurate tracking of the pumping current replication could be achieved in the critical low emissions region of the engine operating curve.
Therefore, it is a primary object of the present invention to reduce the noise and unwanted offset voltages of the EGO output signal by eliminating the need to amplify the noise and offset components by the amplification factor of the differential amplifier 30.